qemu-kvmde 磁盘cache的参数有一下五种，在建立虚拟机的过程中 通过下面的函数设定
int bdrv_parse_cache_mode(const char *mode, int *flags, bool *writethrough)
{
    *flags &= ~BDRV_O_CACHE_MASK;

    if (!strcmp(mode, "off") || !strcmp(mode, "none")) {
        *writethrough = false;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "directsync")) {
        *writethrough = true;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "writeback")) {
        *writethrough = false;
    } else if (!strcmp(mode, "unsafe")) {
        *writethrough = false;
        *flags |= BDRV_O_NO_FLUSH;
    } else if (!strcmp(mode, "writethrough")) {
        *writethrough = true;
    } else {
        return -1;
    }

    return 0;
}
这五种的详细解释如下：其中默认是writeback